琵琶湖北東部,後期白亜紀貝月山花崗岩体

抄録・要旨

The Kaizukiyama granite is distributed northeast of Lake Biwa and was intruded into the Jurassic accretionary complex of the Mino-Tamba Belt．The grantic mass crops out over an area of approximately 120k㎡，(14 km N-S，11.5 km E-W）．lt consists mainly of equigranular coarse-grained hornblende bearing biotite granite to granodiorite，associated with porphyritic hornblende bearing biotite granite-granodiorite，medium-grained muscovite granite，and granite porphyry．
Equigranular hornblende bearing biotite granite-granodiorite of the Kaizukiyama granite consists mainly Plagioclase，K-feldspar and quartz，with subordinate biotite and hornblende，and accessory opaque minerals，apatite，zircon，monazite，allanite and tourmaline．Porphyritic hornblende bearing biotite granite-granodiotite have the same mineralogy，but contain K-feldspar megacrysts up to 7 cm in length．Medium-grained muscovite granite occurs in the central part of the Kaizukiyama granite as schlieren less than a few tens of meters in width，and consists mainly of plagioclase，K-feldspar，quartz and muscovite，with accessory biotite，garnet，opaque minerals，apatite，zircon and monazite．
Color index（modal composition of mafic minerals）and modal plagioclase / K-feldspar ratio of slab samples and magnetic susceptibility tend to decrease from the margin to core，and with increasing of altitude（from 250 m to 1200 m）．
SiO_2 contents of hornblende bearing biotite granite−granodiorite and　muscovitegranite from the Kaizukiyama granite are 66.52-72.23 wt% and 75.25-75.87 wt%，respectively．Muscovite granite plots with in the S-type granite field on the ACF diagram of White and Chappell（1977）．